## Most Common Site of Ulcerative Colitis **Key Point:** Ulcerative colitis characteristically involves the rectum and sigmoid colon in the majority of cases, with proximal extension occurring in only a minority of patients. ### Pattern of Involvement Ulcerative colitis follows a **continuous, retrograde pattern** of inflammation: - Always begins in the **rectum** - Extends proximally in a continuous manner (no skip lesions) - Rectosigmoid involvement occurs in ~90% of cases - Pancolitis (entire colon) occurs in only ~20% of cases ### Distinguishing Features from Crohn's Disease | Feature | Ulcerative Colitis | Crohn's Disease | |---------|-------------------|------------------| | **Most common site** | Rectosigmoid | Terminal ileum | | **Pattern** | Continuous | Skip lesions | | **Depth** | Mucosa + submucosa | Transmural | | **Small bowel involvement** | Never | Common (40%) | | **Ileal involvement** | Only if pancolitis | Very common | **High-Yield:** The rectum is ALWAYS involved in ulcerative colitis. If the rectum is spared, the diagnosis is not ulcerative colitis—consider Crohn's disease or other colitis. **Clinical Pearl:** Patients often present with rectal bleeding and urgency because the rectosigmoid is the most inflamed region. Proctosigmoidoscopy alone may be sufficient for initial diagnosis in many cases. ### Why Other Sites Are Less Common - **Terminal ileum:** This is the hallmark of Crohn's disease, not ulcerative colitis. UC does not involve the small bowel unless pancolitis extends to the ileocecal valve. - **Jejunum:** Never involved in either UC or typical Crohn's disease. - **Cecum and ascending colon:** These are involved only when disease extends proximally from the rectosigmoid (pancolitis), which is less common than isolated rectosigmoid disease.
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